This invention relates to a method for treating an etchant, more specifically a method for treating an etchant including copper (I) chloride or ferric chloride containing copper, in which case, chlorine gas generated therein is used to treat other etchants for the regeneration.
It is generally known that a conductive pattern of e.g., an integrated circuit in a substrate is manufactured by solving copper in areas other than those corresponding to conducting lines to be used with the aid of a solution of copper (II) chloride ferric chloride.
It is desirable to regenerate the waste of etchant and thus to reuse it for other etching Processes from the view point of avoiding the environmental pollution and economic requirements, where the etchant waste contains copper (I) chloride produced in the following etching process: EQU CuCl.sub.2 +Cu.fwdarw.2CuCl,
or the waste is generated from the etching process in which the solution of ferric chloride is used. Several methods have been proposed for regenerating the etchant waste, where copper is withdrawn from the waste and then the etchant is regenerated. Some of the methods have already been applied to practical use.
In one of the most typical methods for regenerating the waste of etchant containing copper (I) chloride, CuCl in the waste is regenerated into copper (II) chloride CuCl.sub.2 with the aid of hydrochloric acid and hydrogen peroxide.
In this method, however, all contents of copper dissolved from the copper foil of the substrate into the etching solution are stored as copper (II) chloride CuCl.sub.2, thereby rapidly giving rise to an excess concentration of CuCl.sub.2.
Accordingly, an excess amount of etchant is usually supplied to a disposal tank in a factory of etching and, therefore, there is a danger of pollution which eventually occurs in the course of the disposal process of the excess etchant or its transportation.
In spite of the above-mentioned treatment with hydrogen peroxide, an improvement for etching has been proposed where the etchant waste is electrolytically treated, so that the etchant is regenerated by changing copper (I) chloride CuCl into copper (II) chloride CuCl.sub.2 with the aid of chlorine generated at the side of the anode in which the waste is transported, and at the same time copper can be electrolytically withdrawn from deposited copper ions as metallic copper at the side of the cathode in which the waste is similarly transported. This method has been disclosed in the Japanese Patent Publication Sho 56-17429, and has already been applied to practical uses.
In this patent publication, the proper adjustment of liquid phase composition in the cathode cell of an electrolytic bath is particularly recommended.
In the method for withdrawing copper on the basis of the electrolytic process according to the Japanese Patent Publication Sho 56-17429, however, complicated operations are required for controlling the liquid phase composition, the respective flow rates of solution supplied to the cathode and anode cells, the balance in pressure, etc, because the liquid phase composition must be kept at a reduced copper concentration of less than 65 g/l for the composite solution of both copper (I) and copper (II) chlorides, under the conditions that the etchant waste is separately supplied into the cathode and anode cells. Moreover, no explicit description is given on the method for treating chlorine gas to be generated; without the treatment, a danger of deteriorating the working environment increases due to the generated chlorine gas.
Moreover, as for the etchant waste resulting from the etching process with a solution of ferric chloride, an electrolytic process is Particularly well known, in which the etchant waste is decomposited in an electrolytic bath having a diaphragm between the anode and cathode cells, so that metallic copper can be obtained from copper ions deposited onto the cathode, and at the same time the ferric chloride can be regenerated by oxidation at the side of the anode.
In such an electrolytic process, the etching solution after the dissolution of copper plates or copper foils in a printed circuit board contains trivalent iron ions, divalent iron ions, divalent copper ions and monovalent copper ions which result from ferric chloride and copper foils. In the course of electrolysis for such an etchant, the reactions of electrolytic reduction occur at the cathode of the electrolytic bath in the following sequence: EQU Fe.sup.3+ +e.sup.- .fwdarw.Fe.sup.2+,
and then, EQU Cu.sup.2+ +2e.sup.- .fwdarw.Cu.sup.+ .fwdarw.Cu.
In other words, ferric chloride is first reduced to ferrous chloride in the solution, and then copper (II) chloride is reduced to copper (I) chloride, thereafter a copper metal is deposited. If, therefore, the electrolysis is continuously performed with a closely circulated apparatus for withdrawing, and at the same time if a part of copper metal deposited onto the cathode, in particular powder of metallic copper fallen out of the surface of the cathode into the solution remains at the bottom, FeCl.sub.3 or CuCl.sub.2 which is newly supplied into the etchant reacts as follows: EQU FeCl.sub.3 +Cu.fwdarw.FeCl.sub.2 +CuCl EQU CuCl.sub.2 +Cu.fwdarw.2CuCl.
Accordingly, the copper which has once been deposited is again dissolved into the solution, thereby reducing the efficiency of copper recovery. In addition, the dissolution provides a considerable amount of CuCl in the regenerated solution. These drawbacks eventually result in a decreased efficiency of etching.
Taking into account these facts, the Japanese Patent Publication Sho 55-18558 has disclosed a method for continuously withdrawing copper by electrolysis of the etchant waste including ferric chloride containing copper and for regenerating the etchant of ferric chloride, in which case the electrolytic reduction process is divided into two steps: In the first step, ferric chloride and copper (II) chloride are reduced to ferrous and copper chlorides, respectively, and, in the second step, metallic copper is deposited.
In the method for withdrawing copper by electrolysis according to the above-mentioned patent publication, however, there are drawbacks due to the complicated installation which permits the reduction of the etchant to be performed just before the electrolytic deposition of copper occurs in the first step, and also due to the difficulty in controlling the liquid phase composition. In addition, similar to the Japanese Patent Publication Sho 56-17429, the method for treating the chlorine gas to be generated is not described. Therefore, there is a danger of deteriorating the working environment due to the resultant gas of chlorine.
Incidentally, if one is restricted only to withdrawing metallic copper from the etchant waste, it is possible to use so called cementation in which iron powder is put into the waste, thereby enabling copper to be reduced on account of the difference in ionization tendency. However, cementation provides an excess content of iron in the solution treated, reuse of the etchant is impossible and the used etchant is abandoned. As a result, this method cannot assure avoidance of pollution in the environment, nor of economic requirements.